Watermark is an item occasionally inserted into a disordered
(sub)stream of timestamped items. The watermark's timestamp value has
the meaning "no items will follow with timestamp less than this." The
value of the watermark is always monotonically increasing in the
(sub)stream, with the effect of superimposing an ordered timestamp
sequence over the original disordered one. Watermark items are used by
windowing processors as anchoring points where the processor knows which
windows it can close and emit their aggregated results.